STYLE: use std:: qualifier on ::abort, ::exit

- consistent with use of <cstdlib> and makes for a stronger distinct
  from Foam::abort etc.

applications/test/alloc/Test-malloc.C

@@ -1,6 +1,6 @@
 #include "stream.h"
 #include <unistd.h>
-#include <stdlib.h>
+#include <cstdlib>
 
 main()
 {

applications/test/alloc/Test.C

@@ -1,4 +1,4 @@
-#include <stdlib.h>
+#include <cstdlib>
 
 class Int
 {

applications/test/maxMem/Test-maxMem.C

@@ -1,5 +1,6 @@
 #include <iostream>
-#include <stdlib.h>
+#include <cstdlib>
+
 using namespace std;
 
 int main(int argc, char *argv[])